Systems and methods for selective cell and/or stack control in a flowing electrolyte battery

ABSTRACT

The invention provides in various embodiments methods and systems relating to controlling energy storage units in flowing electrolyte batteries.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/752,997 filed Apr. 1, 2010, which is a continuation of U.S. patentapplication Ser. No. 10/886,881 filed Jul. 8, 2004, now U.S. Pat. No.7,939,190, which claims benefit of priority to U.S. Provisional PatentApplication Ser. No. 60/485,871, filed Jul. 9, 2003. Each of theaforementioned applications are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates generally to operation of flowing electrolytebatteries. In particular, in one aspect, the invention relates tomethods and systems for controlling, monitoring, charging, and/ordischarging (collectively “controlling”) flowing electrolyte batteries.

BACKGROUND OF THE INVENTION

Blackouts and other power inconsistencies present a problem for powerusers. Even seconds of downtime or minor aberrations in power qualitycan translate into millions of dollars of loss for businesses. TheElectric Power Research Institute (EPRI) has estimated that powerdisturbances cost industry as much as $400 billion a year.

The public utility grid was not designed, nor is it equipped to deliverpower without interruption. It also lacks the ability to modulate,condition and improve the power it delivers—increasing the risk thatcustomers will be subjected to surges, sags and other power qualityinconsistencies. Furthermore, the more than 2.5 million miles ofelectric wire that deliver power from the country's main grids arevulnerable to all types of risk. Severe weather can cause major outages,but even the occasional downed wire or broken pole can threaten to shutdown production, leave workers idle, and/or stop communications.

Alternatives to reliance on a public utility grid include distributedgeneration systems that, once installed at a customer's site, can boostgeneration capacity for continuous and backup power, relievetransmission and distribution bottlenecks, and support power systemmaintenance by generating temporary backup power. Distributed powermodels also offer customers the flexibility to customize their powersystem based on their individual needs, and they are sited and installedin much less time than it takes to conduct conventional central plantsystem power generation upgrades.

Existing alternatives, however, still leave companies with no fullysatisfactory distributed generation system. Fuel cells, for example,require more development before being suited for distributed powergeneration. Other options include solar, wind, reciprocating engines andmicro turbines. All of these options, however, require local energystorage to work effectively. Solar and wind power are energy sources ofopportunity, meaning they are not always available all day every day.Fuel cells and micro-turbines are steady state devices that can make useof natural gas. These technologies, however, do not load follow.Consequently, transients need to be supplied from storage. Use of thesetechnologies, requires the availability of effective and reliablestorage systems.

One type of energy storage system is an electrolyte battery. Such abattery can be configured as an array of stacks of cells (typicallylead-acid cells), with each stack of cells having its own electrolyte.Since each stack is a closed system, the open-circuit voltage (V_(oc))across a stack is indicative of the amount of charge stored in thatparticular stack. Differences in the open-circuit voltages betweenstacks can be used to determine which stacks in the system are fullycharged and which are only partially charged.

A second type of electrolyte battery is a flowing electrolyte battery.One such battery employs an array of stacks of cells, where the stacksshare a common flowing electrolyte. Since the stacks share theelectrolyte, measurements of the open-circuit voltage across a stackonly indicate whether the stack stores some non-zero amount of charge,rather than indicating the stack's state of charge relative to the otherstacks in the system. Moreover, differences in the open circuit voltagesbetween stacks are typically indicative of some internal abnormalitythat has lowered a stack's internal resistance.

For example, in a zinc-bromide flowing electrolyte battery, the stacksshare an aqueous zinc bromide electrolyte and have their own electrodesfor deposit and dissolution of elemental zinc during charge anddischarge cycles. In this type of battery, the electrolyte flow to astack can be inhibited by poorly placed zinc deposits. Additionally,nucleation on the electrodes can cause dendrite formation and branchingbetween cells. In either case, the internal resistance of the affectedstack is lowered, causing a corresponding drop in the open-circuitvoltage across the stack.

Differences in open-circuit voltages between stacks in flowingelectrolyte battery systems can affect the charge and discharge cyclesof the stacks and, potentially, the operation of the battery. Forexample, in the aforementioned zinc-bromide battery, a lowered opencircuit voltage in a particular stack causes an increase in the rate ofzinc accumulation in the faulty stack during the charge cycle and adecrease in the rate of zinc reduction in the faulty stack during thedischarge cycle. Moreover, the additional zinc stored in the faultystack typically comes from the electrolyte normally utilized byneighboring stacks. As a result of the lowered zinc availability, theenergy storage capacity of the neighboring stacks may be reduced.Another consequence is that the stack having the increased zincaccumulation does not fully strip during discharge; eventually resultingin zinc accumulating on the electrodes of the faulty stack to such anextent that it causes internal short circuiting between the cells of thestack. This can potentially destroy the stack and possibly, the entirebattery. A further consequence is that the increased zinc accumulationrestricts the channels through which the electrolyte flows. As theelectrolyte flow acts to cool the stack, the restricted flow may causethe stack to over heat and melt critical components.

Prior art solutions to this problem have involved fully “stripping”i.e., fully discharging, each stack in the battery, completely removingany stored charge from all of the cells in all of the stacks. Ideally,this process eliminates the abnormality that initially caused thedifference in open-circuit voltage between the stacks. For example, afull strip typically dissolves dendrites between plates and/or depositsobstructing electrolyte flow. However, a full strip of a flowingelectrolyte battery is typically time consuming (often taking one or twodays to complete) and may have to be repeated every few days for arecurring problem. A full strip of the battery typically renders itunavailable or at a significantly reduced capacity for electricalapplications, necessitating the purchase and installation of additionalredundant battery systems. Moreover, a full strip is often unnecessarysince typically a minority of the stacks in the battery is operatingabnormally.

Therefore, there is a need for improved methods and apparatus forcontrolling, monitoring, charging and/or discharging cells in a flowingelectrolyte battery.

SUMMARY OF THE INVENTION

The invention addresses the deficiencies in the prior art by providing,in various embodiments, improved, methods, systems and features forcontrolling, monitoring, charging and/or discharging (collectively“controlling”) flowing electrolyte batteries. According to one aspect,the invention addresses the deficiencies in the prior art by providingmethods, systems and features for controlling individual stacks ofbattery cells in a flowing electrolyte battery. In a further embodiment,the invention provides methods, systems and features for controllingindividual battery cells in a flowing electrolyte battery. Among otheradvantages, the invention increases the flexibility with which cellstacks can be charged and stripped; enables regular and ongoing batterymaintenance, without taking the battery offline; maintains the batteryat a predictable and consistent charge capacity; reduces the likelihoodof stack failures due, for example, to electrolyte flow blockage,thermal runaway, and/or dendrite formation; reduces the risk of unevencell plating; increases the number of charge/discharge cycles available;and reduces expenses relating to maintaining redundant battery systems.

In one aspect, the invention provides a systems and methods forindividually controlling cell stacks in a flowing electrolyte batteryhaving a plurality of cell stacks. Preferably, the battery is a flowingzinc bromide battery. However, the invention may be employed with anysuitable flowing electrolyte battery. According to one configuration,the invention includes a stack controller for operable interconnectionto one of a plurality of the cell stacks in the battery. According toone feature, the stack controller controls current flow, individually,through the cell stack. According to one embodiment, the system includesa plurality of stack controllers, with each one being associated foroperable interconnection to an associated one of the plurality of cellstacks. In one preferred configuration, the interconnection between thecell stacks and the stack controllers may be via electricalinterconnection. However, in other configurations, the interconnectionmay be optical, a combination of electrical and optical or any suitabledirect or electrically isolated interconnection approach.

According to an alternative embodiment, rather than having individualcell stack controllers, a single master controller controls theindividual cell stacks. As in the case of the individual controllers,the single master controller controls the current flow to each cellstack on a stack-by-stack basis; thus, providing all of the importantadvantages of the individual stack controllers. In another alternativeembodiment, rather than having a single master controller, a pluralityof controllers, less than the number of cell stacks, control theindividual cells stacks. In a further alternative embodiment, theinvention provides a multilevel stack controller architecture in which,a master controller provides direction to one or more additional stackcontrollers to provide individual stack control.

In some embodiments, current control to each cell stack is substantiallyor completely unaffected by current control provided to another cellstack. However, in some embodiments, current control to a particularstack is allowed to affect current control to another stack, but inpredictable and controllable manner.

In one embodiment, a stack controller provides control signals to one ormore solid state switches to control current flow to (e.g., charging)and/or from (e.g., discharging) a cell stack. According to one approach,the master and/or individual controllers regulate a duty cycle of acontrol signal to the one or more solid state switches to control thecurrent cell stacks.

According to an alternative embodiment, the invention provides anindividual dc/dc converter/controller for each cell stack. Preferably,the dc/dc converter/controller controls current flow to and from thecell stack. According to one feature, each dc/dc converter/controlleroperates substantially or completely independently from each other dc/dcconverter/controller and provides, for example, charging, discharging,electrode plating, electrode stripping, electrolyte flow and cell stackmaintenance control for an associated cell stack. According to anotherfeature, each dc/dc converter/controller provides voltage, current,electrolyte flow, and temperature monitoring for an associated cellstack. According to another feature, the dc/dc converter/controller, inresponse to, for example, an under current, over current, under voltage,over voltage, under charge, over charge, and/or over temperaturecondition, can take an associated individual cell stack off line (e.g.,for maintenance), without substantially affecting operation of thebattery as a whole.

As discussed going forward, the term “stack controller”, may include anyof the above discussed stack controller configurations, including thedc/dc converter/controller or any other suitable controllerconfiguration that enables control of individual cell stacks.

According to another feature, the invention monitors the current througheach cell stack, and based on the measured current, alters the currentbeing directed to or away from the cell stack. In one embodiment, thestack controller calculates an average of the currents through each ofthe cell stacks, and then adjusts the current through particular ones ofthe cell stacks based on how many amperes the monitored current flowdeviates from the calculated average. According to one implementation,the invention provides a threshold current deviation from the averagethat must be exceeded prior to making any adjustment in current flow toa cell stack. By way of example, the invention may require greater thana plus or minus 0.1 A, 0.25 A, 0.5 A, 0.75 A, 1 A, 1.5 A, 2 A, 2.5 A or3. A, deviation from the calculated average, prior to adjusting thecurrent through a particular cell stack.

According to one embodiment, the invention takes a current measurementof all of the cell stacks periodically, calculates the average cellstack current, ranks the currents in order of deviation from theaverage, and schedules the cell stacks for current adjustment based onthe ranking; adjusting those cell stacks with the largest currentdeviation from the average first and progressing through the ones withthe least deviation from the average. In one approach, the inventionadjusts the currents by scheduling a current deprivation, whereas inother approaches, the invention schedules provision of additionalcurrent to deviating cell stacks, and/or provides a combination ofcurrent addition and current deprivation, depending, for example, onwhether the current flow to a cell stack is higher than the average orlower than the average.

In other embodiments, the invention adjusts the currents to multiplecell stacks in a substantially concurrent fashion, and in one particularembodiment, adjusts all of the currents to all of the cell stacks in asubstantially concurrent or simultaneous manner. According to oneconfiguration, the invention adjusts the current flow to a cell stack ina fashion that is linearly dependent on the current deviation from theaverage. However, in other embodiments, other suitable relationships maybe employed.

In another aspect, the invention monitors a subset of the current flowsthrough the cell stacks substantially in real time. The invention mayalso calculate the average of the currents through the cell stacks insubstantially real time. According to a further embodiment, theinvention performs current adjustments in substantially real time, andoptionally, substantially concurrently. The subset of cell stacks mayinclude all of the cell stacks.

According to an alternative embodiment, rather than calculating anaverage current through the cell stacks, the stack controller monitorsthe current flow through a cell stack and adjusts current flow throughthe cell stack, based substantially solely on a deviation from anexpected current flow through the cell stack. In one implementation, theinvention provides a threshold current deviation from the expectedcurrent that must be exceeded prior to making any adjustment in currentflow to the cell stack. By way of example, the invention may requiregreater than a plus or minus 0.1 A, 0.25 A, 0.5 A, 0.75 A, 1 A, 1.5 A, 2A, 2.5 A or 3. A, deviation from the expected current flow, prior toadjusting the current through the cell stack.

According to a further feature, the invention includes hysteresis in thedecision as to whether to make a cell stack current adjustment. Theinvention may also include, for example, a state of cell stack voltage,temperature, electrolyte flow, and/or charge in the decision as towhether to alter current flow to a cell stack.

While in some aspects, the invention particularly excludes the batterycell stacks and the particular devices in series with the cell stacksthrough which the charging/discharging current flows, in other aspects,the invention particularly includes the current flow devices (e.g., thesolid state and/or mechanical switches), and/or the cell stacks and/orthe entire flowing electrolyte battery.

According to another aspect, a stack controller is in communication witha sensor for detecting fault conditions in a particular cell stack and astack controller for altering a charging condition of the cell stack inresponse to a fault condition.

In one embodiment, the sensor includes a voltage sensor for monitoringan open-circuit voltage across one or more of the cell stacks. Inanother embodiment, the sensor includes a current sensor for monitoringthe current entering and/or leaving one or more of the cell stacks. Inanother embodiment, the invention includes a history logger forrecording sensor readings with regard to particular cell stacks.According to another embodiment, the sensor includes an electrolyte flowsensor for monitoring circulating electrolyte in the battery. In oneconfiguration, the electrolyte flow sensor includes a pump sensor fordetecting when an electrolyte pump is pumping. In another embodiment,the invention includes a timer for determining the passage of apredetermined increment of time.

According to one aspect, the invention includes a switch incommunication with or as part of the stack controller, where the switchmodulates a charging current supplied to the stack in response to afault condition. In another embodiment, the invention includes a switchin communication with or as part of the stack controller and a resistiveelement in communication with the switch, where the switch places aresistor across the stack to discharge is in response to a faultcondition or as a way of performing maintenance. In another embodiment,the invention includes a switch in communication with or as part of thestack controller, where the switch can be shorted across the terminalsof the cell stack used to complete a discharge process in response to afault condition or as a mechanism for performing maintenance. In anotherembodiment, the invention includes a switch in communication with or aspart of the stack controller, where the switch can divert currentthrough a resistive element when the current to the stack is interruptedthe current distribution through the other batter stacks will beunaffected.

In another aspect, the invention provides a method for individual cellstack control in a flowing electrolyte battery. According to oneembodiment, in response to detecting a fault condition in an individualcell stack, the method of the invention alters a charging conditionassociated with the cell stack. According to an additional feature, inresponse to detecting correction of the detected fault condition, themethod of the invention again alters a charging condition associatedwith the cell stack.

In one embodiment, the step of altering the charging condition inresponse to the correction of the fault condition includes restoring thecharging condition to its original state. In another embodiment, thestep of detecting the fault condition includes detecting a change in anopen-circuit voltage across the cell stack and/or detecting a change ina current flow to the cell stack.

According to a further embodiment, the step of altering the chargingcondition in response to the detection of the fault condition includesreducing the amount of current charging the cell stack. In anotherembodiment, reducing the amount of current includes applying apulse-width modulation with a duty cycle less than 100% to a circuitcharging the cell stack. In another embodiment, reducing the amount ofcurrent includes altering the current output from a dc/dcconverter/controller to the cell stack.

In one embodiment, the step of altering a charging condition in responseto the detection of the fault condition includes reducing the amount ofcurrent charging a particular cell stack, while maintaining the amountof current charging another one of the cell stacks at a constant. Inanother embodiment, reducing the amount of current to the particularcell stack, while maintaining the amount of current to the other cellstack includes providing charging current to the particular cell stackand the other cell stack for a substantially equal period of time. Inanother embodiment, the step of altering a charging condition inresponse to detection of the fault condition includes substantiallydepleting the particular cell stack of stored energy and subsequentlycreating a short circuit across the particular cell stack to maintain itin an uncharged state.

Other aspects, embodiments, features and elements of the invention willbe discussed in detail below with regard to the illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with regard to the followingillustrative drawings in which like referenced designations refer tolike elements, but in which the elements may not be draw to scale. Itshould be noted that the following drawings are illustrative in natureand are not intended to limit the scope of the invention.

FIG. 1 schematically depicts a flowing electrolyte battery including aplurality of cell stacks of a type that may be employed with anillustrative embodiment of the invention.

FIG. 2 is a schematic block diagram showing an exemplary implementationof a cell stack controller interconnected to a battery of the typedepicted in FIG. 1 according to an illustrative embodiment of theinvention.

FIG. 3 is a flowchart depicting a process for individual stack controlin a flowing electrolyte battery according to an illustrative embodimentof the invention.

FIG. 4 is a state diagram depicting an illustrative state machineimplementation of a process of the type depicted in FIG. 3.

FIG. 5 is a schematic diagram depicting an interconnection between onecell stack and a stack controller according to an illustrativeembodiment of the invention.

FIG. 6 is a state diagram illustrating the operation of the stackcontroller of the type depicted in FIG. 5.

FIG. 7 is a block diagram of a stack controller approach according to analternative illustrative embodiment of the invention.

FIG. 8 is a more detailed schematic diagram of a dc/dcconverter/controller of the type employed in the illustrative embodimentof FIG. 7.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As discussed above in summary, the invention addresses the deficienciesin the prior art by providing, in various embodiments, improved,methods, systems and features for controlling, monitoring, chargingand/or discharging (collectively “controlling”) flowing electrolytebatteries. According to some illustrative embodiments, the inventionaddresses the deficiencies in the prior art by providing methods,systems and features for controlling individual stacks of battery cellsin a flowing electrolyte battery. In other illustrative embodiments, theinvention provides methods, systems and features for controllingindividual battery cells in a flowing electrolyte battery. In otherillustrative embodiments, stack controllers and sensors interconnectedwith individual battery stacks and/or cells provide detection of a faultconditions, and in response to detecting such fault conditions, alterone or more charging conditions of individual battery stacks and/orcells. If necessary, alterations can be repeated and/or iterated and anoperator can be alerted about the fault conditions.

It is to be understood that although the following illustrativediscussion utilizes the terms “battery” and “stack,” the scope of theinvention is not so limited. In a broader sense, the invention enablesthe control of individual charge storage units in an array ofinterconnected charge storage units in a battery, such as a flowingelectrolyte battery, regardless of the terminology used to describe theindividual units or the array. For example, in one illustrativeembodiment, the invention enables the control of individual cell stacksin a battery of interconnected cell stacks. It is this exemplaryembodiment that is developed in the remainder of the illustrativedescription. However, in other illustrative embodiments, the inventioncan be described as enabling the control of individual cell stacks in atower of interconnected cell stacks. In other illustrative embodiments,the invention can be described as enabling the control of individualbattery cells in a stack of battery cells, and so on.

The term “battery” is to be understood to refer to an array of chargestorage units, such as an array of interconnected cell stacks, while a“stack” is to be understood to refer to an individual charge storageunit, such as a stack of battery cells, for which individual control isdesired.

Some of the illustrative embodiments of the invention are employ asingle source of dc current for charging all of the cell stacks in thebattery. These embodiments are described particularly with respect toFIGS. 1-9. However, in other illustrative embodiments, the invention isprovides multiple independent dc current sources; preferably one foreach cell stack. These embodiments are particularly described below withrespect to FIGS. 7 and 8.

FIG. 1 depicts an exemplary flowing electrolyte battery 100 constructedfrom nine cell stacks 104 ¹ . . . 104 ⁹ (generally “cell stacks 104”) ofthe type that may be employed with illustrative embodiments of theinvention. The exemplary battery 100 includes three groups of threestacks 104 electrically connected in parallel—e.g., the stacks 104 ¹,104 ², and 104 ³ form one group of stacks 104 electrically connected inparallel. Together, the three groups of stacks are electricallyconnected in series to form the battery 100. The stack topologyillustrated in FIG. 1 is purely illustrative and selected to facilitatediscussion, since illustrative embodiments of the invention interoperatewith batteries possessing arbitrary internal topologies, stack and cellconfigurations.

In this example, the stacks 104 are hydraulically interconnected (notshown) to permit the sharing of a common, flowing electrolyte. Thisenables the battery 100 to achieve a higher energy density relative to abattery with electrically interconnected stacks that are nothydraulically interconnected, and therefore, have separate, non-flowingelectrolytes. In this example, the battery 100 is a zinc-bromide batteryand the flowing electrolyte is an aqueous zinc bromide solution.

Illustrative embodiments of the invention can be sold separately forintegration with a flowing electrolyte battery, such as the battery 100of FIG. 1. Alternatively, as illustrated in FIG. 2, the invention may beconfigured to be integrated with the flowing electrolyte battery 100 andsold as a single unit. In this illustrative embodiment, each stack 104is electrically connected to a stack controller 200 ¹ . . . 200 ⁹(generally 200). The stack controllers 200 are, in turn, electricallyconnected to a master controller 204.

In the illustrative embodiment of FIG. 2, the interconnections permitpowering of individual stack controllers 200 and the master controller204, as well as exchange of data and/or commands among the individualstack controllers 200, the master controller 204, or both. For example,a regulated dc power supply can utilize the electrical interconnectionsto provide power to the stack controllers 200 and the master controller204, or the master controller 204, itself, can include the functionalityof the power supply and selectively supply power to each individualstack controller 200. The interconnections can include dedicated linesfor exchanging data and/or commands between the various controllers.Additionally, data and power may be provided over the same line. Theexchange of data between controllers can be accomplished using protocolsknown to the art, such as RS-232, I²C or CAN Bus. It is also possible toexchange data and/or commands between controllers using known wirelessprotocols, such as Bluetooth or IEEE 802.11(b). Additionally, asmentioned above, interconnections may be optically isolated, using forexample, fiber optic interconnections.

In the illustrative embodiment of FIG. 2, the master controller 204controls the interface between the battery 100 and an externalapplication that the battery 100 powers—for example, an inverter feedinga shaped 480 VAC, three phase waveform to semiconductor processingequipment. The master controller 204 monitors the power available to thebattery 100. When surplus power is available, the master controller 204charges the battery 100 by providing a charging current to the stacks104. When the power supplied to the battery 100 is insufficient to meetthe requirements of the load, the master controller 204 draws power fromthe stacks 104 and provides it to the load. In one embodiment, themaster controller 204 also detects a sudden demand for power thatexceeds the load's average demand by a predetermined amount and suppliesthe difference instantaneously or nearly instantaneously from thebattery 100. Preferably, the master controller 204 also controlsoperation of electrolyte pumps and numerous other support systems in thebattery, e.g., cooling systems, user interfaces, system telemetry, andthe like.

Internal defects in a cell stack 104 typically result in a loweredinternal resistance in that stack 104; in some configurations drawingcharge current away from its nearest neighbor stacks 104. Therefore, inthe illustrative embodiment of FIG. 2, one function of the stackcontroller 204 is to reduce the magnitude of the current entering afaulty stack 104. Assuming a single dc current source supplies all ofthe cell stacks 104, stripping one stack by reducing the chargingcurrent at the current source entails stripping the entire battery,which requires significant down time, effectively removing the batteryfrom operation as discussed above. By reducing the charge current to afaulty stack 104, while the other stacks 104 continue to charge, theinvention causes the flowing electrolyte to become increasingly reactiveand removes deposited zinc from the faulty stack 104, eliminatingdendrites and other plating defects that can cause a drop in internalresistance and, in turn, open-circuit voltage. This approach effectivelyenables a single stack to be sufficiently stripped to cure a fault,without necessitating taking the battery offline or stripping theremaining cell stacks.

The controllers 200 and 204 may be implemented in any suitable manner.By way of example, in some illustrative embodiments, one or more of thecontrollers 200 and 204 may be a programmed-logic device (PLD), aprogrammable-logic array (PLA), a field-programmable gate array (FPGA),or other specialized hardware device. In other illustrative embodiments,one or more of the controllers 200 and 204 may be a software processexecuting on a single processor, a multiprocessor computer, or adistributed processing array executing an operating system.

FIG. 3 is a simplified flowchart summarizing a process for individuallycontrolling a cell stack in a flowing electrolyte battery according toan illustrative embodiment of the invention. Using this process or asimilar process, stack controllers, such as the stack controllers 200control associated cell stacks, such as the cell stacks 104. Preferably,control includes monitoring the associated cell stacks for conditionsrelevant to stack operation. Such monitoring can include monitoring forfault conditions. However, a plurality of relevant battery operatingconditions, including, without limitation: cell, stack, and/or batteryopen circuit voltage, current in and out, charge capacity, temperature,and/or resistance; under and/or uneven electrode plating; load demand;power grid voltage/status; electrolyte flow status, rate, volume and/orobstructions; electrolyte chemical composition; electrolyte stack leaksfrom a leak sensor; stack weight from, for example, a strain gauge; thestate of one or more pumps circulating the flowing electrolyte in thebattery from, fore example, a pump sensor; and the like. As shown at300, when the system is first activated, the stack controllers areinitialized. After successful initialization, the stack controllersmonitor (step 304) operably connected cell stacks, such as a cell stack104.

In response to a relevant condition being detected at step 304, thestack controller alters an operating condition (e.g., a chargingcondition) of the battery 100 at step 308. For example, in response todetecting an unacceptable deviation in a cell and/or stack voltageand/or current, the stack controller 200 may alter one or more chargingconditions associated with the stack 104. This may include increasing ordecreasing the charging current to a particular cell stack.Additionally, in response to, for example, detecting an unacceptablyhigh temperature or low internal stack resistance, the stack controller200 may take a particular stack offline to avoid thermal run away. Thestack controller may also initiate partial online stripping of aparticular stack. Also, in response to, for example, a decrease in linevoltage, a change in load, or a power grid failure, the stack controller200 may take steps to switch the battery from drawing current forcharging to providing uninterrupted power to the line. In step 312, thestack controller 200 detects a correction or change in the initiallydetected condition in the stack. If the condition persists or is notcorrected, the stack controller 200 may continue to provide the remedialaction of step 308 until the condition changes, or until a predeterminedfailure condition (e.g., a time out) is satisfied. In response todetecting a change in the relevant condition, in step 216, the stackcontroller 200 can again alter one or more battery operating conditions.For example, the stack controller 200 may restore the original chargecondition that existed before detection of the relevant condition atstep 304. Alternatively, the stack controller may bring a stack backonline, or for example, in response to detecting that the power grid isback online, initiate a process to halt the battery from supporting theline and return it to a charging or other quiescent mode.

With particular reference to a zinc-bromide battery 100, a faultcondition can manifest itself as a drop in the open-circuit voltageacross or the charge current through a stack, such as a stack 104. Asdiscussed in greater detail below, according to the illustrativeembodiment, the stack controller remedies the fault by reducing theamount of charge current entering the stack 104. Reducing the chargingcurrent enables the corrosive electrolyte to remove zinc from theelectrodes of the cells included in the stack 104. When enough zinc isremoved to essentially correct the fault condition (e.g., remove adendrite, nucleation feature, over plating or some other electrodeplating anomaly), the correction is detected by an increase in theopen-circuit voltage or a decrease in the charge current entering thestack. In response, the stack controller 204, restores the chargecurrent to its original value. If the fault is not corrected, the stackcontroller 204 can maintain the reduced charge current relative to theother stacks 104 in an attempt to further deplete the stack 104 ofelemental zinc. In one illustrative embodiment, if repeated remedialmeasures (e.g., multiple cycles of depletion) fail to correct thedefect, the stack controller 200 may alert an on-site or off-siteoperator, either directly or indirectly using, for example, the mastercontroller 204.

FIG. 4 depicts a simplified state diagram depicting a current controlprocess in accord with the illustrative embodiment of FIG. 3. The statediagram may be implemented in the stack controller 200, for example, asa programmed logic device (PLD) or as a general purpose or dedicatedprocessor executing the appropriate instructions. After initialization,the stack controller 200 enters a NORMAL state 350. The stack controllerreceives an input, .DELTA.I, reflecting a difference between thecharging current entering the associated stack 104 and a currentthreshold value.

In one illustrative implementation, the threshold current value is afunction of the average current that has entered the particular stack104 over a previous period of time. In another illustrativeimplementation, the threshold value is chosen as an ideal chargingcurrent to be provided to the stack. For example, if the array of stackswas composed of 3 parallel-connected groups of 2 series-connected stacks104 and the charging current provided by the master controller was 100A, the predetermined current value per stack 104 would be about 16 A.According to another illustrative embodiment, .DELTA.I is calculated asa deviation from an actual measured average of the charging currentprovided to all of the cell stacks 104.

According to the illustrative embodiment, if ΔI remains below a selectedfirst value, e.g., less than plus or minus about 0.1 A, 0.25 A, 0.5 A,0.75 A, 1 A, 1.5 A, 2 A, 2.5 A or 3. A, the stack controller 200 remainsin the NORMAL state 350 and does not take action to alter the chargingconditions associated with the stack. If ΔI exceeds the first value butdoes not exceed a second value (e.g., 0.25 A, 0.5 A, 0.75 A, 1 A, 1.5 A,2 A, 2.5 A or 3 A), the controller 200 progresses to the PWM_CHARGEstate 354 under the assumption that there is an incipient problem in thestack 104 that can be corrected by reducing the charging current intothe stack 104. Under this condition, the stack controller 200 may employa variety of techniques to adjust the charging current. In oneconfiguration, the stack controller 200 applies pulse-width modulationwith a duty cycle of less than 100% to the charging current to reducethe overall amount of charge current entering the stack 104. In oneimplementation, the period for the pulse-width modulation is on theorder of about 100 seconds, so as to allow sufficient time for iondiffusion through the flowing electrolyte.

In an alternative implementation, and as discussed below with respect toFIGS. 7 and 8, in state 354, the stack controller may include a dc/dccontroller/converter dedicated to the particular stack to be adjusted.In this implementation, the dc/dc controller/converter adjusts thecurrent being provided to or taken away from the stack 104, independentfrom and without any effect on the remaining stacks. The current to astack with a lowered internal resistance can also be limited byproviding the stack with current for only a particular time period, lessthan the time period for which current is normally provided.

As a result of reducing current to or removing current from the stack104, the stack 104 loses elemental zinc and thus, stored energy from itselectrodes. This eliminates, for example, the dendrite, nucleation, orother uneven plating feature causing a reduction in the internalresistance of the stack. This process is colloquially referred to as“open stripping.” If open stripping successfully completes and thebattery 100 enters either a discharge mode (where it powers the load), afloat mode (where the battery 100 is fully charged and awaitsutilization), or a settle mode (where the controller 200 samples atperiodic intervals the charge current into the battery as it charges),then the controller 200 returns to the NORMAL state 350.

If ΔI exceeds the second value, then the stack controller 200 progressesto the STRIP state 358, concluding that the associated stack 104 isexperiencing a significant problem, such as internal shorting, thatcannot be corrected merely by reducing the charging current entering thestack 104. The controller 200 initiates procedures to shallow or deepstrip the stack 104, which may or may not require taking the battery offline, as discussed in greater detail below. If the stripping processsuccessfully completes and the battery 100 enters either a dischargemode (where it powers a load), a float mode (where the battery 100 isfully charged and awaits utilization), or a settle mode (where thebattery samples at periodic intervals the charge current into thebattery as it charges), then the controller 200 returns to the NORMALstate 350 and returns to monitoring for a change in a relevant batteryoperating condition in step 304.

The state diagram of FIG. 4 reflects the independent operation of thestack controllers 200, each associated with a particular stack 104.According to a feature of the illustrative embodiment, if only oneparticular stack 104 or a minority of stacks 104 in the battery 100 isexperiencing an operational fault, the remaining stack controllers 200continue to command their associated stacks to receive the normal chargecurrent as if there was no problem with the faulty stack 104. In thisway, the illustrative embodiment keeps the battery online and availableto provided backup power if called upon to do so.

FIG. 5 is a schematic block diagram 500 of an exemplary interconnectionbetween a stack controller 200, its associated stack 104 and mastercontroller 204 according to one illustrative embodiment of theinvention. The stack controller 200 operably communicates with thesensors 400, from which it receives one or more sensor measurements asan input. As discussed above, sensor inputs may include, for example,cell, stack, and/or battery open circuit voltage, current in and out,charge capacity, temperature, and/or resistance; under and/or unevenelectrode plating; load demand; power grid voltage/status; electrolyteflow status, rate, volume and/or obstructions; electrolyte chemicalcomposition; ph of electrolyte, electrolyte stack leaks from a leaksensor; stack weight from, for example, a strain gauge; the state of oneor more pumps circulating the flowing electrolyte in the battery from,fore example, a pump sensor; and the like. The stack controller 200 alsocommunicates with the control inputs of the switches 404, 408, 412, 416and 420, through which it can selectively open and close the appropriateswitch and control the charge and discharge of the stack 104. In anotherembodiment, the stack controller 200 can also control the charge anddischarge of the stack 104 by mechanically throttling the flow ofelectrolyte to the stack 104. The master controller 204 alsocommunicates with the switches 404 and 408 and the control terminal ofthe switch 404. The switches of FIG. 5 can take the form of a collectionof discrete components (e.g., relays and/or IGBTs) interconnected on acircuit board, or a collection of power transistors, (e.g., powerMOSFETs, on a single silicon die).

According to one feature, the stack controller 200 either includes or isin communication with a condition history logger, which records the datafrom the sensors over time. According to another feature, the stackcontroller 200 includes or is in communication with a timer that canprovide a system time or a signal indicating the passage of a period oftime.

In response to the stack controller 200 closing the isolator switch 408,the master controller 204 takes control of the assertion of thecontactor switch 404. The contactor switch 404 is normally in acontact-open position, inhibiting the flow of charge current between themaster controller 204 and the stack 104. By asserting the contactorswitch 404, which is typically incidental to charging the stacks 104 inthe battery 100, the master controller 204 provides a necessary, butinsufficient path, to initiate charging the stack 104.

The stack controller 200 controls the assertion of the isolator switch408, the modulator switch 412, the short switch 416, and the dischargeswitch 420. The isolator switch 408 permits stack controller 200 tocontrol whether the master controller 204 can assert the contactorswitch 404. If a fault or other relevant operating condition is detectedin the stack 104 that requires the electrical isolation of the stack104, the stack controller 200 opens the isolator switch 408, preventingthe master controller 204 from creating a path for a charge current tothe faulty stack 104.

The modulator switch 412 is enables the stack controller 200 to regulatethe charging current to the stack 104 through pulse-width modulation, asdiscussed above. By generating a series of rectangular pulses, with theappropriate duty cycle, and applying them to the control terminal ofmodulator switch 412, the charge current provided to the stack 104 ispulse-width modulated, without substantially affecting, or onlyaffecting in a predictable manner, the charge current provided to anyother stack 104. The diode 424 enables the stack 104 to provide power toa load on the battery 100 when the switch 412 is open. Moreparticularly, the modulator switch 412 requires a finite amount of timeto change state, which can cause a delay in supplying power from thestack 104 to a load. The diode 424 is reverse-biased in normaloperation—i.e., during charging of the stack 104—but becomes forwardbiased in the event that the master controller 204 attempts to drawpower from the stack 104, permitting the circumvention of an openmodulator switch 412 until the modulator switch 412 has had sufficienttime to close and establish a path for the outflow of current from stack104 to the load.

The short switch 416 enables deep discharge of stack 104. In the eventthat a full strip of the entire battery 100 has been ordered, the stackcontroller 204 first either opens or operates at a low duty cycle themodulator switch 412 to enable the corrosive electrolyte to strip thestack 104, or shallow strips the stack by engaging the discharge switch420, as discussed below. When the amount of stored energy remaining inthe stack 104 is sufficiently small that shallow or open strippingrequires a significant amount of time, closing the short switch 416causes a short circuit across the terminals of the stack 104,facilitating the removal of the remaining stored energy.

The discharge switch 420 permits the stack controller to slowly stripthe stack. Closing the discharge switch 420 places a power resistor 428in parallel with the stack 104, significantly reducing the amount ofcurrent received by the stack 104 relative to the reduction of currentavailable through operation of the modulator switch 412.

In considering the above described illustrative embodiment, it should benoted that the functionality of the invention can be differentiallyallocated between the stack controllers 200 and the master controller204 in various embodiments. For example, in the illustrative embodiment,the stack controllers 200 individually implement the fault detection andcharging functionality associated with each individual stack 104, whilethe master controller 204 controls the charging of the stacks 104 in thebattery 100 as a whole. Such an embodiment is useful for applicationsthat value distributed control and increased fault tolerance.

In another illustrative embodiment, the stack controllers 200 areessentially conduits for sensor measurements from the stack 104 to themaster controller 204. In turn, the master controller 204 makesoperational decisions concerning the charging and discharging ofindividual stacks 104 based on the provided data. Such an embodiment maybe preferred, for example, when it is possible to integrate thefunctionality of stack controllers 200 and master controller 204 on asingle integrated circuit, which can result in significant cost savings.

In a further illustrative embodiment, the master controller 204 merelyreports commands received from an outside operator using a userinterface, such as a control panel, or a network link, or othertelecommunications connection. In this embodiment, in addition to thefunctionality described above, the stack controllers 200 are also dc/dcconverters. In contrast to the previously discussed system, which hadonly one dc current source for charging all the stacks 104 and relied onthrottling the dc current at the individual stacks 104 to effectuatestripping, this embodiment features one dc current source per stack 104.This enables the selective charging and discharging of any individualstack 104 in the battery, regardless of the charge or discharge state ofthe other stacks 104. In one version of this embodiment, stackcontrollers 200 maintain historical sums of the currents entering andleaving their associated stacks 104 as indicated by associated chargesensors 400. If any of these historical sums are negative, theappropriate stack controller 204 will charge the appropriate associatedstack 104. This illustrative embodiment is discussed in further detailwith respect to FIGS. 7 and 8.

FIG. 6 is a state diagram depicting an exemplary operation of the stackcontroller 200 as interconnected in the illustrative embodiment of FIG.5 in response to various operating conditions detected in the stack 104and the battery 100 by the sensors 400. The logic implementing thisstate diagram may be programmed in the stack controller 200. In thisembodiment, there is no sensor that directly provides the state of thecontactor switch 404 or the master controller 204 to the stackcontroller 200. However, the stack controller 200 can indirectly detectthe mode of operation of the master controller 204 by monitoring thestate of an electrolyte pump subsystem (not shown). Before the mastercontroller 204 charges or depletes the stacks 104 in the battery 100, itactivates the pump subsystem to circulate the flowing electrolytebetween stacks 104. If the stack controller 200 detects activity in thepump subsystem, it knows that the master controller 204 is preparing tosource or sink current from the battery 100. Otherwise, it knows thatthe battery is either charging or “floating,” i.e., standby in a fullycharged state.

At initialization, the stack controller 200 enters the POWERUP1 state450. The modulator switch 412, isolator switch 408 discharge switch 420and short switch 416 are all open. After the passage of about 100milliseconds, the stack controller 200 enters the POWERUP2 state 454.The modulator switch 412 is closed to provide a necessary connection tocharge the stack 104, which itself is insufficient until the isolatorswitch 408 is also closed.

After about another 100 milliseconds, the stack controller 204 entersthe WAIT_PUMP state 458. The modulator switch 412 and isolator switch408 are closed, permitting the master controller 204 to direct chargingcurrent to the stack 104 upon the closing of the contactor switch 404.The stack controller remains in the WAIT_PUMP state 458 until a pumpsensor 400 indicates that the pumps circulating the electrolyte areactive, at which time the stack controller enters the PWM_CHARGE state462. In the PWM_CHARGE state 462, the isolator switch 408 is closed andthe modulator switch 412 can be either open or closed, as the stackcontroller 204 applies pulse-width modulation to the charging current,as discussed above.

If the pump subsystem is disabled and the average stack current into thestack 104 is below a first threshold value (as discussed above withrespect to FIG. 4), then the stack controller transitions from thePWM_CHARGE state 462 to the FLOAT state 466. In the FLOAT state 466, thestack 104 is substantially fully charged and idles until current isrequired from it. In the FLOAT state 466, the modulator switch 412 isopen, while the isolator switch 408 remains closed. If the modulatorswitch 412 is closed and the pump subsystem is activated, after about100 milliseconds, the stack controller 200 returns to the PWM_CHARGEstate 462.

If, while in the PWM_CHARGE state 462, the average current into thestack drops below a second threshold value less than the first thresholdvalue discussed above—e.g., about 0.1 A in one implementation, then thestack controller 200 transitions to the DISCHARGE state 470. In theDISCHARGE state 470, the modulator switch 412 and the isolator switch408 are both closed. If the pump subsystem is idle and the averagecurrent into the stack 200 remains below the first threshold value, thenthe stack controller 204 enters the FLOAT state 466. If the stack 104begins to source current in excess of a third value (e.g., greater thanabout 1 A, 1.25 A, 1.5 A, 1.75 A, 2 A, 2.5 A, 3 A, or 3.5 A) then thestack controller 200 reenters the PWM_CHARGE state 462.

If, while in the PWM_CHARGE state 462, an amount of time passes inexcess of the period for the pulse-width modulation discussed above (inone embodiment about 60 seconds), then the stack controller 200transitions to the SETTLE state 474. The modulator switch 412 and theisolator switch 408 remain closed. If the pump subsystem is inactive andthe average current into the stack 200 is less than the first thresholdvalue, then the stack controller 200 enters the FLOAT state 466, asdiscussed above. If the average current into the stack 200 is less thanthe second threshold value, then the stack controller enters theDISCHARGE state 470. If a predetermined period of time passes (in oneembodiment about 55 seconds) without either of these transitionsoccurring, then the stack controller transitions to the READ_CURRENTstate 478, before transitioning back to the PWM_CHARGE state 462. If thestack 104 is brought online to supply power to a load (i.e., peaksharing mode) more than ten times while the stack controller is inPWM_CHARGE state 462, then the stack controller 200 enters the STRIPstate 482, and forces the master controller 204 to initiate a strip ofthe entire battery 100, alerting an operator to a fault condition.

FIG. 7 is a schematic diagram of an individual stack control system 500employing a dedicated dc/dc converter/controller 502 ₁ through 502 ₂₇(collectively 502) to control each of 27 battery stacks 504 ₁ through504 ₂₇ (collectively 504). Each of the battery stacks 504 are connectedwith each other in parallel and include 54 cells. The output voltagefrom the dc/dc converter/controllers 502 is nominally about 550 Vdc andis provided to an inverter 506 to supply the load with 580 VAC threephase. A rectifier 510 rectifies the 480 Vac three phase voltage fromthe power grid 512 and provides it to the dc/dc converter/controllers504 for charging the battery stacks 508. A master battery controller 508communicates with each of the dc/dc converter/controllers 502, forexample, over a CAN bus, to exchange information and to provide controlcommands and sensor information to the dc/dc converter/controllers 502.The dc/dc converter controllers 502 can perform all of the functionalityof the various illustrative stack controllers described above. Also,like the previously described illustrative embodiments, functionalitymay be divided between the dc/dc converter/controllers 502 and themaster battery controller 508 in any suitable way.

As described below in more detail with respect to FIG. 8, the dc/dccontroller/converters 502 are bidirectional and include both a buckconverter section and a boost converter section. The buck convertersection provides charging current to an associated battery stack 504,while the boost converter section provides power to the load duringdischarge. As also described in more detail with respect to FIG. 8, andas in the case of the previously described embodiments, one feature ofthe system 500 is that the master controller 508 tracks the state of allof the battery stacks 504 with respect to their availability to providebackup power to the load. More particularly, the master controller 508tracks, for example, the state of charge of each of the stacks 504,which ones are offline for servicing, and which ones are available butnot at full capacity. The master controller also tracks substantially inreal time the load requirements (e.g., current being drawn by the load)and the state of the grid (e.g., the voltage level of the grid). Basedon this information, the master controller regulates when and to whatdegree the battery capacity is switched to support the load. Anadvantage of this feature enables the battery controller of theinvention to match the support provided by the battery to the supportactually required by the load during an uninterruptible power supply(UPS) event.

An advantage of the configuration of FIG. 7 is that each of the dc/dccontroller/converters 502 control an associated individual battery stack504, independently from any of the other dc/dc converter/controllers 502controlling their associated battery stack 504. More particularly, thisenables each of the dc/dc converter/controllers 502 to charge,discharge, take on- and offline, and partially or fully strip anassociated battery stack 504, with negligible effect on any of theremaining battery stacks 504. Additionally, the preferred 27 stackbattery configuration provides enough stacks and enough capacity thatone or more stacks 504 may be taken offline, for example, for deepstripping or other service, without affecting the availability of thebattery to provide backup power. The variously above described thresholdvoltages may also be employed with the dc/dc converter controllerconfiguration of FIG. 7.

FIG. 8 provides a more detailed schematic diagram of an exemplary dc/dcconverter/controller 502 of the type depicted in FIG. 7. Moreparticularly, as shown in FIG. 8, the dc/dc converter/controllers 502include a bi-directional dc/dc converter 600 for dedicated associationwith a battery stack 504. The bidirectional dc/dc converter 600 convertsa dc link voltage (shown as 550 Vdc in FIG. 7) from, for example, therectifier 510 of FIG. 7, to a current that charges the stack 504. It canalso discharge the stack 504 by taking power from the stack 504 andconverting it to a current that feeds the dc link voltage, for example,to an inverter, such as the inverter 506 of FIG. 7. In each case, ittranslates from one voltage to another. A local controller 602 controlsthe dc/dc converter/controller operation. The local controller 602accepts commands from and exchanges information with a master batterycontroller, such as the controller 508 of FIG. 7. These commandsinclude, for example, commands to charge and discharge the stack 504,and the magnitudes of such charging and discharging. The mastercontroller 508 can also provide preload information to the localcontroller 602 regarding how much current will be required by the loadin the event of a UPS event. A UPS event is detected by the loss of thegrid. When the grid collapses, the dc link voltage drops. The greaterthe load being supplied the faster the dc link voltage drops. The amountof current required from each dc/dc converter/controller 502 andassociated battery stack 504 is dependent on the total load and thenumber of battery stacks 504 that are online and available to deliverpower. There may be a time when an individual stack is being stripped orfaulty and not available to deliver power. Thus, the master controller508 knowing the total load requirements and the number of stacks 504that are available, can preset the current required from each stack 504if in the next instant a UPS event occurs. This enables each dc/dcconverter/controller 502 to respond with the appropriate current commandto its associated stack 504 in response to a UPS event being detected.

The local controller 602 interfaces with a power switch 607 by a gatedrive interface circuit 603, which conditions the signals from a digitalcontrol level at the local controller 602 to the appropriate voltage andcurrent levels for the upper and lower switches Q1 and Q2, respectively,of the power switch 607. The local controller 602 also providesisolation between the upper and lower switches Q1 and Q2. The powerswitch 6072 connects to the stack 504 through a choke (e.g., aninductor) 604. When the stack 504 is being charged, the upper switch Q1is pulse width modulated by the local controller 607 and the gate drive603. A current sensor 605 provides current feedback to the localcontroller 607. The local controller 607 varies the duty cycle of thepulse width modulated signal to the upper switch Q1 to maintain adesired current. When the stack 504 is being discharged, the lowerswitch Q2 is pulse width modulated by the local controller 607 and thegate drive 603, causing current to flow from the stack 504 to the dclink and thus charging the capacitor 610. According to the illustrativeembodiment, a conventional boost configuration is employed to boost thevoltage from the stack 504 to the dc link. As previously described, thedc link voltage is applied to an inverter, such as the inverter 506 ofFIG. 7. A voltage sensor 606 and a current sensor 605 provide feedbackto the local controller 602 so that it can control the current andvoltage of the dc link during battery discharge.

A second voltage sensor 611 measures the voltage of the stack 504 andreports back to the local controller 602, which preferably also providesthe information to the master controller 508, so that state of charge aswell as any fault conditions may be determined. Each sensor has a rangeof appropriate values associated with each battery condition. Any valuethat is outside of the range may be indicative of a fault condition andappropriate corrective action is performed.

Accordingly, the invention provides in various embodiments improvedmethods and systems for controlling flowing electrolyte batteries,preferably as individual stacks of battery cells.

What is claimed is:
 1. A method for individual stack control in aflowing electrolyte battery including at least a first battery cellstack and a second battery cell stack interconnected with each other andsharing a common flowing electrolyte, the method comprising the stepsof: individually controlling a charging condition of the first batterycell stack, at least by regulating a duty cycle of a first controlsignal to one or more first switches, based at least in part oninformation regarding an operating condition of the first battery cellstack; and individually controlling a charging condition of the secondbattery cell stack, at least by regulating a duty cycle of a secondcontrol signal to one or more second switches, based at least in part oninformation regarding an operating condition of the second battery cellstack.
 2. The method of claim 1, wherein: the step of individuallycontrolling the charging condition of the first battery cell stackcomprises increasing the charging condition of the first battery cellstack; the step of individually controlling the charging condition ofthe second battery cell stack comprises decreasing the chargingcondition of the second battery cell stack while continuing to chargethe second battery cell stack; and the steps of individually controllingthe charging condition of the first battery cell stack and individuallycontrolling the charging condition of the second battery cell stack areperformed simultaneously.
 3. The method of claim 1, wherein: the step ofindividually controlling the charging condition of the first batterycell stack comprises holding the charging condition of the first batterycell stack substantially constant; the step of individually controllingthe charging condition of the second battery cell stack compriseschanging the charging condition of the second battery cell stack; andthe steps of individually controlling the charging condition of thefirst battery cell stack and individually controlling the chargingcondition of the second battery cell stack are performed simultaneously.4. The method of claim 1, wherein: the step of individually controllingthe charging condition of the first battery cell stack comprisesstripping the first battery cell stack; the step of individuallycontrolling the charging condition of the second battery cell stackcomprises controlling the charging condition of the second battery cellstack without stripping the second battery cell stack; and the steps ofindividually controlling the charging condition of the first batterycell stack and individually controlling the charging condition of thesecond battery cell stack are performed simultaneously.
 5. The method ofclaim 1, wherein the operating condition of the first battery cell stackis selected from the group consisting of current flow in the firstbattery cell stack, voltage of the first battery cell stack, presentcharge capacity of the first battery cell stack, temperature of at leasta portion of the first battery cell stack, an internal resistance of thefirst battery cell stack, and electrolyte leak information regarding thefirst battery cell stack.
 6. The method of claim 1, wherein theoperating condition of the first battery cell stack includes electrodeplating rate in the first battery cell stack.
 7. The method of claim 1,wherein the operating condition of the first battery cell stack includesa load demand for a load for which the flowing electrolyte battery istasked with providing power.
 8. The method of claim 1, wherein theoperating condition of the first battery cell stack includes a state ofa primary power source to a load for which the flowing electrolytebattery is tasked with providing power.
 9. The method of claim 1,wherein the operating condition of the first battery cell stack includesa status of electrolyte flow to the first battery cell stack.
 10. Themethod of claim 1, wherein the operating condition of the first batterycell stack includes a chemical composition of an electrolyte availableto the first battery cell stack.
 11. The method of claim 1, wherein theoperating condition of the first battery cell stack includes stackweight for the first battery cell stack.